STPM10 datasheet(15/51 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

Doc ID 17728 Rev 3

15/51

Theory of operation

7.1

General operation description

The STPM10 is capable of performing measurements of active, reactive and apparent

energy, RMS and instantaneous voltage and current values, and line frequency information.

Most of the functions are fully programmable using internal configuration bits accessible

through the SPI interface. The STPM10 works as a peripheral in microcontroller-based

metering systems. The ZCR and WDG pins are used to provide zero-crossing and watch-

dog information, and the SPI pins are used to communicate with the microcontroller.

The STPM10 includes volatile internal registers that hold the useful information for the

metering system. Two kinds of active energy are available: wide-band active energy (AW)

which includes all harmonic content (also called type 0) and fundamental active energy

(AF), limited to the 1st harmonic (also called type 1). This latter energy value is obtained by

filtering type 0 active energy. Both the two active energies are stored in up-down counting

accumulator registers with a 20-bit length. Reactive and apparent energies are also

available with a 20-bit accumulation.

The STPM10 also provides the RMS values for voltage and current. Due to the modest

dynamic variation of the voltage, the RMS value is stored with a resolution of 11 bits, while

the RMS current value has a resolution of 16 bits. The instantaneous (momentary) sampled

value of voltage and current are also available with a resolution of 11 and 16 bits,

respectively. The line frequency value is stored with a resolution of 14 bits.

Due to the proprietary energy computation algorithm, the STPM10 calibration is quick and

simple, allowing calibration at only one point over the entire current range.

The configuration and calibration parameters must be downloaded in the internal non-

volatile memory of STPM10 at power-up.

7.2

Analog inputs

Input amplifiers

The STPM10 has one fully differential voltage input channel and two fully differential current

input channels.

The voltage channel consists of a differential amplifier with a gain of 4. The maximum

differential input voltage for the voltage channel is ± 0.3 V.

The two current channels are multiplexed (see Chapter 7.9 for details) to provide a single

input to a preamplifier with a gain of 4. The output of this preamplifier is connected to the

input of a programmable gain amplifier (PGA) with possible gain selections of 2 and 8. The

total gain of the current channels are then 8 and 32. The gain selections are made by writing

to the gain register, and they can be different for the two current channels. If the tamper

function is not used, the secondary current can be disabled.

The maximum differential input voltage is dependent on the selected gain, in accordance

with Table 6.

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